https://leetcode.cn/problems/add-binary/
class Solution {
public:
    string addBinary(string a, string b) {
        string ret;
        int  t=0,cur1=a.size()-1,cur2=b.size()-1;
        while(cur1>=0||cur2>=0||t)
        {
            if(cur1>=0)
            { 
                t+=a[cur1]-'0';
                cur1--;
            }
            if(cur2>=0)
            {
                t+=b[cur2]-'0';
                cur2--;
            }
            ret+=t%2+'0';
            t/=2;
        }
        reverse(ret.begin(),ret.end());
        return ret;
    }
};